Water used in industrial processes and cleaning operations is frequently contaminated with oil, grease, and similar substances and discharged as waste. Particularly in such service industries as automotive and truck repair and servicing operations, water is often used to flush oil and grease from the work area and equipment, and heretofore, it has been the usual practice to dump the effluent into natural streams or rivers or to discharge the effluent directly into the local public drainage or sewer system. With increasing concern for the environment, dumping of contaminated water has been substantially halted, and also, it has been found to be equally undesirable to discharge the waste water directly into sewer systems. Although sewage is frequently treated to remove pollutants so that the water can eventually be reused, the heavy concentration of oil and grease in waste water from industrial cleaning operations makes it difficult to process the effluent in typical treatment facilities so that quality standards can be met. Accordingly, it has become necessary to purify the waste water produced in commercial operations so as to remove substantially all of the oil from the waste water before it is discharged into a public sewage system. In fact, applicable governmental regulations now require removal of oil from waste water in many instances, and more particularly, the Environmental Protection Agency has promulgated regulations establishing standards for acceptable oil content in waste water prior to discharge of such waste water into a public treatment system.
Various attempts have been made heretofore to remove oil, grease, and other similar substances from waste water by making use of the differences in density of oil and water to permit gravitational separation of the oil and water to facilitate removal of the oil from the surface of the water. For example, as illustrated in U.S. Pat. No. 3,862,039 to Summers and U.S. Pat. No. 2,782,929 to Colket, a typical oil-water separator includes a settling tank or compartment into which waste water is admitted and collected for separation and removal of the oil from the water. While the waste water is within the settling tank, the waste water gradually separates into oil and water constituents with the less dense oil floating to the upper surface of the waste water. The upper layer or portion of the waste water, being the most heavily laden with oil, is then removed from the body of the waste water by a "skimming" operation.
In oil-water separators of the type disclosed in the Summers and Colket patents, removal of the oil is accomplished by positioning a drain or outlet at a fixed elevational location within the settling tank so as to continuously drain or skim the uppermost layer of waste water from the body of the waste wear. Consequently, the drain skims substantial quantities of water off the waste water along with the oil, because of variations in the concentration of oil present in the waste water and/or variations in the rate at which waste water is admitted to the settling tank. This removal of water intermixed with oil presents petroleum with collection and storage of the oil thus removed, because it frequently happens that such large quantities of water are removed with the oil that storage capacity for the oil is quickly exhausted, thus requiring frequent drainage of the oil storage facility. Additionally, if the oil storage facility is not drained but allowed to fill to capacity, oil can back up within the oil-water settling tank and eventually pass through the same with the water.
Other attempts to remove oil from the waste water appear to have increased the problem of substantial quantities of water remaining mixed with the oil. For example, the positioning of more than one drain within a settling tank or compartment, as shown in U.S. Pat. No. 4,119,541 to Makaya, has been proposed to reduce the oil content in the processed water, but even greater amounts of water were removed with the oil, because the upper layer of waste water was skimmed into all the drains. Likewise, it has been proposed to provide a second settling tank or compartment into which the water from the first tank is admitted. The second settling tank also includes an oil drain, and both drains are positioned so that the upper layer of waste water is continuously drained during the processing of the waste water. However, while additional oil is removed in the second settling tank, large quantities of water are still drained with the oil, presenting the storage problems mentioned above.